A common problem with joint arthroplasty procedures, such as knee arthroplasty in which the articulating surfaces of the knee joint are resected and replaced with implants, is that the implants often do not optimally cover the resected surfaces of the patient's anatomy. Typical implant sets only provide a few size options, and, given the variability of bone and other anatomy morphology from individual to individual, the closest size for the particular patient will often result in the outer periphery of the implant either over-hanging the resected surfaces, and/or not entirely covering portions of the resected surfaces. Either situation is undesirable and sometimes can lead to problems, such as soft tissue irritation or functional compromise.
Many times in total knee arthroplasty (TKA), poor post-operative patient outcomes are not caused from a poorly-designed prosthesis. Instead, the problem may often stem from a well-designed prosthesis being installed in a less-than-optimal biomechanic position relative to the natural anatomy of the patient in an attempt to get the best anatomic fit. In other words, the probability of revision knee surgery due to pain or abnormal wear may be high even with a well-designed knee-prosthesis if said prosthesis is misaligned or if the prosthesis is installed without considering the biomechanic effects of prosthetic orientation.
In total knee arthroplasty (TKA), a surgeon may determine the size of a femoral knee implant component by measuring the A-P width of the distal femur from an anterior coronal plane to a posterior coronal plane. Bone sizing is done to determine the closest size femoral component without notching the anterior femoral cortex. Due to noticeable gaps in A-P width between sizes of femoral components within a particular orthopedic product portfolio, the biomechanic fit, feel, and function of the implant is compromised in three different ways for three different techniques, respectively.
First, if a surgeon decides to use a posterior referencing technique, the anterior flange of the femoral component implant will fall where it may depending on the anterior-posterior size of the implant. In many cases, a patient's bone size falls between the sizes dictated by an orthopaedic product offering. While providing better bone coverage, using a larger sized implant with a posterior referencing technique can lead to patella stuffing, retinacular stretch, patello-femoral ligament stretch, quadriceps and patellar tendon over-stretching, quad inefficiency, and anterior knee pain due to increased forces on the patella. Conversely, using a smaller sized implant with a posterior referencing technique might cause loose quadriceps and patellar tendons, patellar subluxation, poor patellar tracking, and knee joint instability/laxity.
Second, if a surgeon decides to use an anterior referencing technique, one or more posterior condyles of the femoral component(s) will fall where they may depending on their sizes and geometries. In many cases, a patient's bone size falls between the sizes dictated by an orthopaedic product offering. While providing better bone coverage, using a larger sized implant with an anterior referencing technique can lead to increased collateral ligament tension, a tight-joint in flexion, decreased range of motion, and increased risk of injury to soft tissues such as the ACL and PCL. Conversely, using a smaller sized implant with an anterior referencing technique might cause joint laxity in deep flexion, loose collateral ligaments, and pseudo-patellar baja if a thicker tibial insert is used to compensate for the laxity in flexion.
Third, while it is uncommon to do so, if a surgeon decides to take a middle-of-the-road technique (i.e., arbitrarily referencing somewhere between anterior referencing and posterior referencing), there may be a combination of the aforementioned disadvantages, or there may be a more ideal implant position than what is chosen based purely upon anatomic fit and non-kinetic intra-operative ligament balance.
Software programs that simulate in-vivo functional activities (e.g., LifeMOD/KneeSIM, a product of LifeModeler, Inc. of San Clemente, Calif.), have been used for the purpose of evaluating the performance of implant designs. Such programs use a three-dimensional, dynamics-oriented, physics-based modeling methodology. While these programs have been used to design implant geometries in the past, the prior art has not utilized such software to fine-tune the anatomical placement or sizing of implants (i.e., standard and custom) so that they meet and exceed an individual patient's needs. U.S. Pat. No. 8,078,440 issued to Otto et al. on Dec. 13, 2011 discloses a system and method that uses such software programs for preoperatively characterizing an individual's biomechanic function in preparation of implanting a prosthesis. U.S. Pat. No. 8,078,440 is herein incorporated by reference in its entirety.